1. Field of the Invention
The present invention relates to an image transmission apparatus and, more particularly, to an apparatus for transmitting image data via a transmission path or interface with a low data transmission rate.
2. Related Background Art
As a conventional apparatus for recording information corresponding to a natural image on a recording medium in the state of an analog signal, an electronic still video camera, a camera built-in type video tape recorder, and the like are known.
However, in recent years, since digital information equipments such as personal computers, and the like have become popular, a demand has arisen for a digital image input apparatus which can directly input information corresponding to a natural image to these digital equipment in the state of a digital signal.
FIG. 1 is a schematic block diagram showing the arrangement of a conventional digital image input system.
Referring to FIG. 1, the image input system comprises an image input unit 1 for inputting an image signal, an A/D converter 2, a signal processing circuit 3, a transmission path 4, a signal processing circuit 5 which is arranged at the reception side of a signal transmitted via the transmission path 4 and has a memory capable of storing digital image data for one field, a CPU 6 of a computer, a display interface 7 for outputting a signal to a display device, a display device 8 having a display unit such as a CRT, a liquid crystal display, or the like, a recording unit 9 such as a digital video tape recorder, a digital video disk device, or the like, which stores image data on a recording medium, a command signal line 10 for transmitting a command signal, which is output from the CPU 6 to instruct, e.g., the start of the recording operation, to the recording unit 9, and an operation unit 11 used by an operator to input, e.g., the start command of the recording operation to the CPU 6.
Note that the signal processing circuit 5, the CPU 6, the display interface 7, the display device 8, and the operation unit 11 are arranged as a personal computer system 50.
The conventional image input system will be described below with reference to FIG. 1.
The operation executed when an analog image signal is input, an image represented by the input analog image signal is displayed on the display unit of the display device 8 in the personal computer system 50, and an operator arbitrarily records his or her desired scene of the input analog image signal on the recording medium while confirming the contents of the image, in FIG. 1, will be described below.
Referring to FIG. 1, the image input unit 1 inputs an analog image signal to the system.
As an image signal input to the image input unit 1, for example, when a video signal compatible with the NTSC television system is to be input, a video signal having 60 fields per second is input; when a video signal compatible with the PAL television system is to be input, a video signal having 50 fields per second is input.
Of the video signal input by the image input unit 1, a video signal for, e.g., one field is sampled by the A/D converter 2 at a sampling rate of about 640 samples in the horizontal direction.times.480 samples in the vertical direction, and each sample is analog-to-digital converted to have 8-bit or higher gradation characteristics for each of R, G, and B colors, thus forming digital image data. The formed digital image data is always supplied to the recording unit 3 via the signal processing circuit 3, and is modulated by the circuit 3 to have a signal format (e.g., an FM-modulated signal) suitable for transmission along the transmission path 4. Furthermore, the digital image data is subjected to transmission rate conversion processing by the circuit 3 to match the transmission rate of the transmission path 4. Thereafter, the digital image data is transmitted to the personal computer system 50.
As the transmission path 4, a transmission path with a low transmission rate such as a versatile serial interface such as an RS-232C interface, a telephone line, and the like is normally used. For this reason, the transmission rate conversion processing performed by the signal processing circuit 3 is one for lowering the transmission rate of the digital image data supplied from the A/D converter 2.
The transmitted digital image data is subjected to processing, e.g., demodulation or the like, in the signal processing circuit 5 in the personal computer system 50, and is temporarily stored in the internal memory of the signal processing circuit 5. Thereafter, the digital image data is read out from the memory, and is converted by the display interface 7 to have a signal format such as R, G, and B signals, that can be displayed by the display device 8. The converted image data is displayed on the display unit such as a CRT, a liquid crystal display, or the like, of the display device 8.
As described above, the display device 8 of the personal computer system 50 displays an image represented by the analog image signal input to the system, and the operator confirms the contents of the image represented by the image signal input to the system on the basis of the displayed image on the display device 8. The operator then operates the operation unit 11 to supply a start command of the recording operation for recording the currently input image signal on the recording medium to the CPU 6. The CPU 6 outputs a command signal for instructing the start of the recording operation, and the signal is input to the recording unit 9 via the command signal line 10.
Note that the recording unit 9 always receives digital image data from the signal processing circuit 3, which data is one before being converted to have a signal format suitable for transmission along the transmission path 4. The recording unit 9 records the digital image data supplied from the signal processing circuit 3 in accordance with the command signal transmitted from the CPU 6.
Since the digital image data recorded on the recording medium by the recording unit 9 is one before being converted to have a signal format suitable for transmission along the transmission path 4, it is a signal having a large information volume per field and a high transmission rate. The recording unit 9 has a performance capable of recording digital image data for one field in 1/60 seconds at high speed.
The above-mentioned conventional digital image input system allows the operator to record an image signal representing a required scene of the input image signal on the recording medium while confirming the contents of the input image signal. Therefore, in order to allow the contents of the image signal input to the system to be confirmed on the display device of the personal computer, the input image signal is converted into a format suitable for transmission along a versatile transmission path with a low transmission rate, and the converted signal is supplied to the personal computer system via the versatile transmission path.
Therefore, when a color dynamic image signal having a large information volume per field and a high transmission rate is input to the system, since the versatile transmission path has a very low transmission rate that can transmit only an image signal for one field in, e.g., 5 seconds, an image displayed on the display device of the personal computer is an unnatural image (e.g., the color still image is switched every 5 seconds), far from a dynamic image. For this reason, even when the operator observes the displayed image, he or she cannot accurately confirm the contents of the image signal currently input to the system.
Since the operator cannot accurately confirm the contents of the image signal currently input to the system even when he or she observes the image displayed on the display device of the personal computer system, he or she cannot give a recording operation start instruction to the recording unit at an adequate timing, and cannot arbitrarily record an image signal corresponding to his or her desired scene on the recording medium.
In the conventional digital image input system, for example, when a video camera is connected to the image input unit, a picked-up image signal output from the video camera is input to the system, and the operator arbitrarily records an image signal corresponding to his or her desired scene on the recording medium while confirming the image picked up by the video camera, the operator cannot operate viewfinder functions such as setting and confirmation of the framing, zoom ratio, and focusing state of an image picked up by the video camera, setting and confirmation of the layout of the picked-up image, and the like on the display screen of the display device in the personal computer system in real time.
In order to match the image display rate with the transmission rate of the image signal, a special-purpose transmission path having a high transmission rate may be used in place of the versatile transmission path, or a compression processing circuit, which can reduce the information volume at high speed without impairing the image quality of an image signal, may be used, and the input image signal may be subjected to compression processing by the compression processing circuit so as to transmit the image data with a small information volume. However, in any case, the cost increases, and the system itself becomes very expensive.